kconfig: fix savedefconfig with choice marked optional

savedefconfig failed to save the correct minimal config
when it encountered a choice marked optional.

Consider following minimal configuration:
$cat Kconfig
choice
	prompt "choice"
	optional

config A
	bool "a"

config B
	bool "b"

endchoice

$cat .config | grep -v ^#
CONFIG_A=y

$conf --savedefconfig=defconfig Kconfig

would before this fix result in an empty file, because
kconfig would assume that CONFIG_A=y is a default value.
But because the choice is optional the default is that
both A and B are =n.

Fix so we handle optional choices correct.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>